1. Field of the Invention
The present invention relates to a feedback method and apparatus for multiple base stations multiple input multiple output (MIMO) in a wireless communication system supporting single base station MIMO communication and multiple base stations MIMO communication.
2. Discussion of the Related Art
In the following description, a method of transmitting and receiving feedback information in consideration of a downlink channel status of a user equipment is explained.
In a communication system, a receiving stage enables efficient communication in a manner of transmitting feedback information on a signal received from a transmitting stage. One example of MIMO communication system for the transmitted feedback information is schematically described as follows.
FIG. 1 is a block diagram for transmitting and receiving stages of a general MIMO communication system and information fed back from the receiving stage.
Referring to FIG. 1, a scheduler 101 of a transmitting stage 10 assigns a transmission signal to a proper communication resource using feedback information received from a receiving stage. Subsequently, the transmission signal is transmitted on a channel via an encoding & mapping 102, an MIMO encoding 103 and an orthogonal frequency division multiplexing (OFDM) modulation 104. Meanwhile, the receiving stage 20 receives the transmitted signal and then performs a process reverse to that of the transmitting stage on the received signal. In particular, the received signal undergoes an OFDM modulation 105, an MIMO decoding 106 and a decoding & demapping 107 to obtain transmitted information. Moreover, the receiving stage 200 performs a channel estimation 108 using the OFDM demodulated signal, uses the estimated channel response for the MIMO decoding 106 and for obtaining feedback information 109 to transmit to the transmitting stage. The feedback information is configured as user information 11 and is fed back to the transmitting stage 10, as shown in FIG. 1.
In this case, the information fed back by the receiving stage 20 is classified into OFDM relevant feedback information and MIMO relevant feedback information and is described as follows.
First of all, an OFDM system feeds back channel quality information (hereinafter abbreviated CQI) measured by a user equipment hereinafter abbreviated UE) and channel status information (e.g., SINR (signal to interference and noise ratio), SNR (signal to noise ratio), etc.) to a base station (hereinafter abbreviated BS). Based on the fed-back information, the BS determines resource allocation and MCS (modulation and coding scheme) level.
MIMO-OFDM systems can be classified into an open loop (hereinafter abbreviated OL) type and a closed loop (hereinafter abbreviated CL) type. In OL-MIMO, a UE reports the same feedback information of a conventional OFDM system to a BS. On the contrary, in CL-MIMO, a UE transmits such supplementary information used for downlink transmission as channel information, a weight, a PMI (precoding matrix index) (or, a preferred PMI), a phase for cyclic delay diversity (CDD) scheme and the like to a BS.
The following description is made on the basis of codebook based CL-MIMO. First of all, a UE estimates a PMI most suitable for a current channel status. A CQI has a value differing according to the estimated PMI. Generally, a UE selects a PMI providing a best CQI from PMIs applied to the same channel and then reports a corresponding CQI value to a BS.
Moreover, in an MIMO system, it is able to determine the number of ranks applicable according to a status of channel. For this, a UE is able to inform a BS how many ranks enable most efficient transmission in a current channel status of the UE.
Besides, as mentioned in the foregoing description, the MIMO system can operate in OL or CL. The MIMO system normally selects to use the OL in fast status or the CL in slow status. For this, a UE selects a mode of the OL or CL and then informs a BS of the selected mode.
Meanwhile, the MIMO system is able to enhance sector throughput and cell-edge throughput via multi-BS cooperative signaling. In doing so, if the MIMO system supports multi-BS MIMO operation, how to configure control information for controlling a feedback transmitted by a BS is crucial.